Electronic measurement and testing systems use relays to route analog signals. Switching devices used in these systems are required to have a very high off-resistance and a very low on-resistance. MOS analog switches have the disadvantage of non-zero leakage current and high on-resistance.
An example of a prior art microswitch is illustrated in FIG. 1A at 10. The basic structure is a micromechanical switch that includes a source contact 14, a drain contact 16, and a gate contact 12. A conductive bridge structure 18 is attached to the source contact 14. As shown in FIG. 1B, the bridge structure 18 overhangs the gate contact 12 and the drain contact 16 and is capable of coming into mechanical and electrical contact with the drain contact 16 when deflected downward. Once in contact with the drain contact 16, the bridge 18 permits current to flow from the source contact 14 to the drain contact 16 when an electric field is applied between the source and the drain. Thus, the voltage on the gate 12 controls the actuation of the device by generating an electric field in the space 20. With a sufficiently large voltage in the space 20, the switch closes and completes the circuit between the source and the drain by deflecting the bridge structure 18 downwardly to contact the drain contact 16.
Switches of this type are disclosed in U.S. Pat. No. 4,674,180 to Zavracky et al., the whole of which is incorporated by reference herein. In this device, a specific threshold voltage is required to deflect the bridge structure so that it may contact the drain contact. Once the bridge comes into contact with the drain contact, current flow is established between the source and the drain.
During operation, hysteresis can arise if the voltage required to draw the end of the beam into contact with the drain contact is greater than that required to hold it in contact with the drain. Thus, two modes of operation exist--a hysteretic mode and a non-hysteretic mode. In a hysteretic mode, when the switch is closed, the gap between the beam and the gate is reduced and therefore the gate voltage required to maintain the beam in its downward deflected state is less than the gate voltage required to actuate the switch. To release the beam so that the beam returns to its open state requires a reduction in the gate voltage to a level below not only the gate voltage required to deflect the beam, but also less than the gate voltage required to maintain the beam in its deflected position. A non-hysteretic mode of operation occurs when the switches are employed. The switches have a minimum gate actuation voltage approximately equal to the maximum gate release voltage due in part to the longer beam length and larger gate area.
Another consideration is that the drain end of the switch may also experience an electrostatic force for high drain/source voltages. Increasing the drain/source voltage above a critical value will cause an unstable operation of the device. This effect is the equivalent of breakdown in a solid state device. To obtain consistent performance the source must always be grounded, or the driving potential between the source and the gate must be floating relative to the source potential. However, this arrangement is not acceptable for many applications.
Several microrelays have been described in the prior art. U.S. Pat. No. 5,278,368 to Kasano et al. discloses an electrostatic relay having an insulated beam. Kasano utilizes a gate contact disposed above the beam with a source contact disposed below the beam. With this arrangement, the beam can be deflected downward to provide electrical connection between two contacts. The manufacture of such a device requires the construction and alignment of several layers of conductors and insulators. Additional conductors are disposed above and below the beam, and as such, the drain contact is not part of the electric field generation mechanism. In this arrangement, the beam is connected to an insulator to provide an electric field generation mechanism.
Several investigators have reported the application of micromachining to the fabrication of mechanical switches and microrelays. Petersen (IBM J. Res. Dev. 23 376-85 (1979)) reported the use of bulk micromachined silicon dioxide cantilevers as relay prototypes. In this work, the cantilever is suspended over an anisotropically etched cavity in the bulk silicon, and a plated metal beam attached to the end of the cantilever makes electrical contact when the beam is pulled down electrostatically.
Zavracky and Morrison reported the first surface micromachined switches. These devices were two terminal devices with a resistor placed between the contacts. This arrangement permits the actuation of the device and source current from the same supply.
Hosaka et al. (Proc IEEE MEMS Workshop '93, Fort Lauderdale, Fla., 12-7 (1993) have developed micromechanical multicontact relays that are electromagnetically actuated. The use of electromagnetic actuation restricts the extent to which the device can be miniaturized. The size limitation also places a limit on the switching speed achievable by such devices.
Sakata et al. (Proc. IEEE MEMS Workshop '89, Salt Lake City, Utah, 149-51 (1989)) reported a micromechanical relay using a silicon cantilever pivoted at its middle, suspended over a cavity anisotropically etched in bulk silicon.
Drake at al. (Transducers '95 Eurosensors IX, Stockholm, Sweden (1995)) have reported using a polysilicon bridge structure as the switching element. The bridge is suspended over a cavity etched in the silicon substrate. The electrodes are deposited and patterned on a separate wafer, and the two wafers are bonded together.
Gretillat et al. (J. Micromech. Microeng. 5, 156-160 (1995)), have used a polysilicon/silicon nitride/polysilicon bridge as the mechanical element. The bridge is released from the insulating silicon nitride substrate by surface micromachining.
Yao and Chang (Transducers '95 Eurosensors IX, Stockholm, Sweden (1995)) have reported a device using a silicon dioxide cantilever, released by surface micromachining from a semi-insulating GaAs substrate.